Channel quality indicator method, associated base station, and associated user equipment

ABSTRACT

The method includes generating one or more Reference Signals associated with the one or more Channel Quality Indicators, and includes mapping the one or more Channel Quality Indicator-Reference Signals to the last symbol of the second slot of the one or more subframes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending application Ser. No.13/257,462 filed on Sep. 19, 2011, which is a National Stage ofPCT/JP2010/055144 filed on Mar. 17, 2010, which claims foreign priorityto Australian Application No. 2009901196 filed on Mar. 19, 2009. Theentire content of each of these applications is hereby expresslyincorporated by reference.

TECHNICAL FIELD

The present invention relates to wireless communications systems, andmore particularly to a method for determining and transmitting ChannelQuality Indicator Reference Signals (CQI-RS) from one or more subframessuch that an associated User Equipment (UE) can use the CQI-RS tomeasure CQI.

BACKGROUND ART

In advanced mobile communication systems, such as theLong-Term-Evolution (LTE) system and the Long-Term-Evolution Advanced(LTE-A) system, User Equipment (UE) is utilised to measure and to reporta number of parameters in the communication system including RankIndicator (RI), Channel Quality Indicator (CQI) or Precoding MatrixIndicator (PMI) to the evolved Node B (eNB) thereby enabling support ofresource allocation, link adaptation and spatial multiplexingtransmission.

Currently, LTE (Release-8) RI, CQI/PMI measurement is performed based onthe cell-specific reference signals (CRS). Each CRS is associated withtransmit antenna ports at the eNB (there is a maximum of 4 transmitantenna ports). Therefore, the maximum number of transmission layersthat can be supported for spatial multiplexing is limited by the numberof antenna ports available (i.e. 4).

It is envisaged that for LTE-A (Release-10), the number of antenna portsused for spatial multiplexing or the number of transmission layersshould be up to 8. Therefore, more Reference Signals are needed toenable the support of higher-order MIMO transmission.

Further, a new technology under consideration for LTE-A is CoordinatedMulti-Point (CoMP) transmission. The LTE-A UE may therefore also berequired to measure and report the RI, CQI/PMI (or similar metric) forthe Reference Signal transmitted from the eNBs that participate in CoMPtransmission.

A problem with this increase in complexity is the possibility ofinterference to signals important for basic system operation togetherwith backward compatibility issues on older UEs.

It would therefore be desirable to provide a method which will work withfuture versions of LTE-A, be backwards compatible and alleviateinterference to signals for basic system operation.

It will be appreciated that a reference herein to any matter which isgiven as prior art is not to be taken as an admission that that matterwas, in Australia or elsewhere, known or that the information itcontains was part of the common general knowledge as at the prioritydate of the claims forming part of this specification.

DISCLOSURE OF THE INVENTION

A improved channel quality indicator method for determining andtransmitting one or more Channel Quality Indicator Reference Signalsfrom one or more subframes such that an associated User Equipment canuse the Channel Quality Indicator Reference Signals to measure ChannelQuality Indicator, the subframes including first and second slots, eachof the first and second slots including a plurality of symbols, and eachof the first and second slots forming a resource block, wherein themethod comprising:

-   -   generating one or more Reference Signals associated with the one        or more Channel Quality Indicators;    -   mapping the one or more Channel Quality Indicator-Reference        Signals to the last symbol of the second slot of the one or more        subframes.

The following description refers in more detail to the various featuresand steps of the present invention. To facilitate an understanding ofthe invention, reference is made in the description to the accompanyingdrawings where the invention is illustrated in a preferred embodiment.It is to be understood however that the invention is not limited to thepreferred embodiment illustrated in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a subframe having two normal CyclicPrefix (CP) resource blocks illustrating the location of the CQI-RS forone layer;

FIG. 1B is a schematic diagram of a subframe having two extended CyclicPrefix (CP) resource blocks illustrating the location of the CQI-RS forone layer;

FIG. 2 is a schematic diagram of a subframe having two normal CyclicPrefix (CP) resource blocks illustrating the location of the CQI-RS formultiple layers for multiplexing via (Frequency Division Multiplexing)FDM;

FIG. 3 is a schematic diagram of a subframe having two normal CyclicPrefix (CP) resource blocks illustrating the location of the CQI-RS formultiple layers for multiplexing via hybrid FDM and (Code DivisionMultiplexing) CDM;

FIG. 4 is a schematic diagram of a subframe having two normal CyclicPrefix (CP) resource blocks illustrating the location of the CQI-RS formultiple layers for CoMP cells multiplexed via hybrid FDM and CDM;

FIG. 5 is a schematic diagram of a series of subframes illustrating useof a cell-specific subframe offset;

FIG. 6 is a schematic diagram of a series of subframes illustrating useof a cell-specific subframe offset designed for CoMP cells;

FIG. 7 is a schematic diagram of bandwidth of subframes illustrating theuse of the resource block offset parameter RB_(offset); and

FIG. 8 is a schematic diagram of bandwidth of subframes illustrating theuse of the resource block offset parameter RB_(offset) suitable for CoMPcells.

CARRYING OUT THE INVENTION

Exemplary embodiments of the present invention are next described indetail with reference to the accompanying figures

Referring now to FIG. 1A, there is shown a subframe 100 having twonormal Cyclic Prefix (CP) resource blocks 105, 110. The subframe 100 isshown with a frequency (f) axis and a time (t) axis. The resource blocks105, 110 are transmission units which are one slot 130, 135 wide in time(t) and twelve subcarriers wide in frequency (f). Included in each ofthe slots 130, 135 are seven symbols along the time axis for a normalCyclic Prefix resource block 105, 110. A number of resource elementswhich make up the overall resource block 105, 110 are cell-specificreference signals (CRS) 125 and first and second “Long TermEvolution—Advanced Channel Quality Indicator-Reference Signal” (LTE-ACQI-RS) 115, 120.

In operation, the CQI-RS of a layer is transmitted in last OFDM symbol(i.e. OFDM symbol number 6 in the second slot 135), in order to avoidcollision with Rel-8 cell-specific reference signals (CRS), Rel-8Dedicated Reference Signal (DRS), and Physical Broadcast CHannel (PBCH)and synchronisation signals. Preferably, there are two CQI-RS REs withina resource block 105, 110 and the CQI-RSs are uniformly distributed overthe 12 subcarriers of the resource block. Providing two CQI-RS REs foreach layer is advantageous since it has been found to provide a goodbalance between CQI-RS overhead and CQI measurement performance.

Also shown in FIG. 1A, is a first cell-specific subcarrier offsetf_(offset) for higher-layer configurations. First f_(offset) determinesthe Resource Element (RE) location offset of the CQI-RS from the lowestsubcarrier index in a resource block. This is shown in FIG. 1A for Firstf_(offset)=2. In the preferred case of two CQI-RS REs per resourceblock, First f_(offset) can take value from 0-5.

FIG. 1B is identical to FIG. 1A but illustrates a subframe 100 whichincludes two extended Cyclic Prefix (CP) resource blocks 105, 110. Thesubframe 100 is shown with a frequency (f) axis and a time (t) axis. Theresource blocks 105, 110 are transmission units which are one slot 130,135 wide in time (t) and twelve subcarriers wide in frequency (f). Eachof the slots 130, 135 are six symbols along the time axis for anextended Cyclic Prefix resource block 105, 110. In operation, the CQI-RSof a layer is transmitted in last OFDM symbol (i.e. OFDM symbol number 5in the second slot 135).

Advantageously, by designing CQI-RS for all layers applicable to LTE-Aoperation to be placed in only one particular OFDM symbol within asubframe provides a simple way to avoid interference to/from Rel-8 CRS,Rel-8 DRS, and PBCH and synchronisation signals.

FIG. 2 is shows a subframe 200 having two normal Cyclic Prefix (CP)resource blocks 205, 210 and further shows the preferred location of theCQI-RS for multiple layers for multiplexing via Frequency DivisionMultiplexing. Like FIGS. 1A and 1B, the subframe 200 is shown with afrequency (f) axis and a time (t) axis. The resource blocks 205, 210 aretransmission units which are one slot 230, 235 wide in time (t) andtwelve subcarriers wide in frequency (f). Each of the slots 230, 235include seven symbols along the time axis for a normal Cyclic Prefixresource block 205, 210. A number of resource elements make up theresource block 205, 210 including cell-specific reference signals (CRS)225 together with first LTE-A CQI-RS 240 (layer 1), second LTE-A CQI-RS245 (layer 1), first LTE-A CQI-RS 250 (layer 2), second LTE-A CQI-RS 255(for layer 2), first LTE-A CQI-RS 260 (layer 3), second LTE-A CQI-RS 265(layer 3), first LTE-A CQI-RS 270 (layer 4) and second LTE-A CQI-RS 275(layer 4).

In FIG. 2, CQI-RS of all layers for LTE-A operation are transmitted inthe same OFDM symbol (i.e. symbol number 6) for the case that the layersare multiplexed via FDM. The particular arrangement within the FDMframework is illustrative, other arrangements are possible.

FIG. 3 shows a subframe 300 having two normal Cyclic Prefix (CP)resource blocks 305, 310 and further shows the preferred location of theCQI-RS for multiple layers for multiplexing via hybrid FrequencyDivision Multiplexing (FDM) and Code Division Multiplexing (CDM). Anumber of resource elements make up the resource block 305, 310including cell-specific reference signals (CRS) 325 together with firstLTE-A CQI-RS 315 (layer 1 and layer 2), second LTE-A CQI-RS 320 (layer 1and layer 2), first LTE-A CQI-RS 340 (layer 3 and layer 4) and secondLTE-A CQI-RS 345 (layer 3 and layer 4).

In FIG. 3, CQI-RS of all layers for LTE-A operation are transmitted inthe same OFDM symbol (i.e. symbol number 6) for the case that the layersare multiplexed hybrid via FDM and CDM. The particular arrangementwithin the hybrid FDM and CDM framework is illustrative, otherarrangements are possible.

FIG. 4 shows a subframe 400 having two normal Cyclic Prefix (CP)resource blocks 405, 410 illustrating the location of the CQI-RS formultiple layers for CoMP cells multiplexed via hybrid FDM and CDM. Inoperation, the CQI-RS of a layer is transmitted in last OFDM symbol(i.e. OFDM symbol number 6 in the second slot 435), in order to mitigateCQI-RS intercell interference. The intercell interference is furtherreduced by including a first cell-specific subcarrier offset Firstf_(offset) and a second cell-specific subcarrier offset Secondf_(offset). First f_(offset) determines the Resource Element (RE)location offset of the CQI-RS from the lowest subcarrier index of aresource block for Cell-1. This is shown in FIG. 4 for Firstf_(offset)=2. Second f_(offset) determines the Resource Element (RE)location offset of the CQI-RS from the lowest subcarrier index of aresource block for Cell-2. This is shown in FIG. 4 for Secondf_(offset)=4. Therefore, LTE-A CQI-RS are as follows: first LTE-A CQI-RS440 (layer 1 and 2 for cell 1), second LTE-A CQI-RS 445 (layer 1 and 2for cell 1), first LTE-A CQI-RS 450 (layer 3 and 4 for cell 1), secondLTE-A CQI-RS 455 (layer 3 and 4 for cell 1), first LTE-A CQI-RS 460(layer 1 and 2 for cell 2), second LTE-A CQI-RS 465 (layer 1 and 2 forcell 2), first LTE-A CQI-RS 470 (layer 3 and 4 for cell 2) and secondLTE-A CQI-RS 475 (layer 3 and 4 for cell 2).

Advantageously, f_(offset) allows for robust intercell interferencemanagement for CoMP CQI-RS transmission.

Transmission Period Configuration of LTE-A Only CQI-RS

FIG. 5 is a schematic diagram of a series of subframes 500 illustratinguse of a cell-specific subframe offset SFoffset 510 and the CQI-RStransmission period, T_(CQI-RS).505. T_(CQI-RS).505 is the same as theCQI/PMI reporting period for LTE Rel-8, i.e. 2 ms, 5 ms, 10 ms, 20 ms,40 ms, 80 ms and 160 ms for Frequency Division Duplex (FDD), and 1 ms, 5ms, 10 ms, 20 ms, 40 ms, 80 ms and 160 ms for Time Division Duplex(TDD). However, T_(CQI-RS).505 is cell-specific while the CQI/PMIreporting period is UE-specific, hence the configuration ofT_(CQI-RS).505 and CQI/PMI reporting period are independent. Inpractice, the CQI/PMI reporting period is generally not shorter thanT_(CQI-RS).505.

Higher-layer configured cell-specific subframe offset SFoffset 510determines the subframe offset for CQI-RS transmission relative tosubframe 0 within a frame. SFoffset takes the value from 0 ms to(TCQI-RS-1) ms. FIG. 5 shows a T_(CQI-RS).505 of 2 ms and SFoffset of 1ms. The final resource block of subframes 1 and 3 are shown as element515.

Advantageously, T_(CQI-RS).505 is useful in controlling the CQI-RSoverhead whereas SFoffset 510 is useful for mitigating CQI-RS intercellinterference among CoMP cells. FIG. 6 shows a series of subframes 600and illustrates an example of how SFoffset can be used to avoid CQI-RSof different CoMP cells being transmitted in the same subframe. In thiscase Cell-1 SFoffset 625 has a value of 1 ms and Cell-2 SFoffset 610 hasa value of 0 ms and a T_(CQI-RS).605 of 2 ms. The final resource blockof subframes 0 and 2 are shown as element 620; and the final resourceblock of subframes 1 and 3 are shown as element 615.

Resource Block Allocation for LTE-A Only CQI-RS

The CQI-RS subband which may be denoted k is defined in the similar wayas the CQI-reporting subband for LTE Rel-8. The CQI-RS subband size orequivalently the total number of resource blocks that contain CQI-RS isdetermined based on the system bandwidth for a single component carrier,similar to the CQI-reporting subband size determination for LTE Rel-8.Specifically, the CQI-RS subband size is determined as shown in Table 1.

TABLE 1 CQI-RS Subband Size k vs. System Bandwidth of a single componentcarrier System Bandwidth of a single component CQI-RS Subband carrierSize, k 6-7 Entire system bandwidth  8-10 4 11-26 4 27-63 6  64-110 8

There is only one resource block in a CQI-RS subband that containsCQI-RS. With this in mind, FIG. 7 shows a schematic diagram of bandwidth(20 Mhz) of subframes 700 (having eight resource blocks in each subband715) illustrating the use of the resource block offset parameterRB_(offset) 710. Each subband 715 includes a resource block 705 whichcontains CQI-RS (the subband size=8 resource blocks). The exact locationof the resource block that contains CQI-RS is determined by theparameter RBoffset 710. RBoffset ranges from 0 to k-1.

RBoffset 710 can be either configured by a higher-layer or can cyclefrom the first resource block to the last resource block within thesubband as subframe number increments (i.e. round-robin allocation ofthe CQI-RS to the resource blocks within the subband).

Advantageously, the parameter RBoffset can also be used to mitigateCQI-RS intercell interference among CoMP cells as shown in FIG. 8. InFIG. 8 there shown a Cell-1 RBoffset 820 and a Cell-2 RBoffset 825within a subband 815. The two offsets are used to avoid CQI-RS ofdifferent CoMP cells being transmitted in the same resource block. Incase of the round-robin assignment, collision can be avoided byconfiguring different starting position for different CoMP cell for theround-robin operation.

Advantageously, there is only one resource block in a CQI-RS subbandthat contains CQI-RS. The total number of resource blocks that containCQI-RS is determined based on the system bandwidth for a singlecomponent carrier. Advantageously, the resource blocks containing CQI-RSare uniformly distributed over the system bandwidth which means it isable to cover the entire system bandwidth (within a component carrier).This is known as the “wideband” requirement in LTE-A. In a furtheradvantage, the arrangement minimises the impact on legacy User Equipment(e.g. LTE Rel-8) by minimising the number of resource blocks thatcontains CQI-RS within a subband.

Although the exemplary embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible withoutdeparting from the scope of the present invention. Therefore, thepresent invention is not limited to the above-described embodiments butis defined by the following claims.

This application is based upon and claims the benefit of priority fromAustralian provisional patent application No. 2009901196 filed on Mar.19, 2009 the disclosure of which is incorporated herein in its entiretyby reference.

The invention claimed is:
 1. A method implemented in a base station usedin a wireless communications system, comprising: transmitting to a userequipment (UE) two channel quality indicator (CQI) reference signalsusing two resource elements per transmission layer of a plurality oftransmission layers within a subframe comprising a resource block,wherein each transmission layer of the plurality of transmission layersis multiplexed using frequency division multiplexing; and receiving fromthe user equipment a report determined according to said two CQIreference signals; wherein a value of a subframe offset corresponding tothe subframe is within a range from 0 ms to (T_(CQI-RS)—-1) ms, whereT_(CQI-RS) denotes a CQI reference signal transmission period, and thereport comprises a rank indicator, a channel quality indicator, and aprecoding matrix indicator, wherein: the subframe comprises the resourceblock and a last orthogonal frequency division multiplexing symbol inthe resource block conveys at least one of the two CQI referencesignals; the CQI reference signal transmission period is cell-specificand a CQI reporting period or a PMI reporting period is userequipment-specific; the two channel quality indicator reference signalsare repeated at a period of 5 subframes, 10 subframes, 20 subframes, 40subframes, or 80 subframes; and the CQI reference signal transmissionperiod is 1 ms, 5 ms, 10 ms, 20 ms, 40 ms, or 80 ms.
 2. The methodaccording to claim 1, wherein the CQI reporting period or the PMIreporting period is equal to or longer than the CQI reference signaltransmission period.
 3. The method according to claim 1, wherein thevalue of the subframe offset is greater than 0 ms and up to(T_(CQI-RS)-1) ms, where T_(CQI-RS) denotes a CQI reference signaltransmission period.
 4. The method according to claim 1, wherein thesubframe offset is cell-specific.
 5. The method according to claim 1,wherein said two CQI reference signal resource elements are used for oneor more antenna ports for spatial multiplexing, the number of said oneor more antenna ports being equal to or less than 8, or for one or moretransmission layers, the number of said one or more transmission layersbeing equal to or less than
 8. 6. The method according to claim 1,wherein a CQI reference signal position depends on a cyclic prefix (CP)length.
 7. The method according to claim 1, wherein the base station isconfigured to be used in a Coordinated Multi-Point (CoMP) transmission.8. The method according to claim 1, wherein the value of the subframeoffset is greater than 0 ms and up to (T_(CQI-RS)-1) ms, whereT_(CQI-RS) denotes the CQI reference signal transmission period, and thevalue of the subframe offset is taken to avoid channel quality indicator(CQI) reference signals being transmitted in the same subframe.
 9. Abase station used in a wireless communications system, comprising: atransmitter to transmit to a user equipment (UE) two channel qualityindicator (CQI) reference signals using two resource elements pertransmission layer of a plurality of transmission layers within asubframe comprising a resource block, wherein each transmission layer ofthe plurality of transmission layers is multiplexed using frequencydivision multiplexing, and the CQI reference signal transmission istransmitted from the base station at a CQI reference signal transmissionperiod; and a receiver to receive from the user equipment a reportdetermined according to the two CQI reference signals; wherein a valueof a subframe offset is within a range from 0 ms to (T_(CQI-RS)-1) ms,where T_(CQI-RS) denotes a CQI reference signal transmission period, andthe report comprises a rank indicator, a channel quality indicator, anda precoding matrix indicator, wherein: the subframe comprises theresource block and a last orthogonal frequency division multiplexingsymbol in the resource block conveys at least one of the two CQIreference signals; the CQI reference signal transmission period iscell-specific and a CQI reporting period or a PMI reporting period isuser equipment-specific; the two channel quality indicator referencesignals are repeated at a period of 5 subframes, 10 subframes, 20subframes, 40 subframes, or 80 subframes; and the CQI reference signaltransmission period is 1 ms, 5 ms, 10 ms, 20 ms, 40 ms, or 80 ms.
 10. Amethod implemented in a user equipment (UE) used in a wirelesscommunications system, comprising: receiving from a base station twochannel quality indicator (CQI) reference signals using two resourceelements per transmission layer of a plurality of transmission layerswithin a subframe comprising a resource block, wherein each transmissionlayer of the plurality of transmission layers is multiplexed usingfrequency division multiplexing; and transmitting to the base station areport determined according to said two CQI reference signals; whereinthe value of the subframe offset is within a range from 0 ms to(T_(CQI-RS)-1) ms, where T_(CQI-RS) denotes a CQI reference signaltransmission period, and the report comprises a rank indicator, achannel quality indicator, and a precoding matrix indicator, wherein:the subframe comprises the resource block and a last orthogonalfrequency division multiplexing symbol in the resource block conveys atleast one of the two CQI reference signals; the CQI reference signaltransmission period is cell-specific and a CQI reporting period or a PMIreporting period is user equipment-specific; the two channel qualityindicator reference signals are repeated at a period of 5 subframes, 10subframes, 20 subframes, 40 subframes, or 80 subframes; and the CQIreference signal transmission period is 1 ms, 5 ms, 10 ms, 20 ms, 40 ms,or 80 ms.
 11. The method according to claim 10, wherein the CQIreporting period or the PMI reporting period is equal to or longer thanthe CQI reference signal transmission period.
 12. The method accordingto claim 10, wherein the value of the subframe offset is greater than 0ms and up to (T_(CQI-RS)-1) ms, where T_(CQI-RS) denotes the CQIreference signal transmission period.
 13. The method according to claim10, wherein the subframe offset is cell-specific.
 14. The methodaccording to claim 10, wherein said two CQI reference signal resourceelements are used for one or more antenna ports for spatialmultiplexing, the number of said one or more antenna ports being equalto or less than 8, or for one or more transmission layers, the number ofsaid one or more transmission layers being equal to or less than
 8. 15.The method according to claim 10, wherein a CQI reference signalposition depends on a cyclic prefix (CP) length.
 16. The methodaccording to claim 10, wherein the base station is configured to be usedin a Coordinated Multi-Point (CoMP) transmission.
 17. A user equipment(UE) used in a wireless communications system, comprising: a receiver toreceive from a base station two channel quality indicator (CQI)reference signals using two resource elements per transmission layer ofa plurality of transmission layers within a subframe comprising aresource block, wherein each transmission layer of the plurality oftransmission layers is multiplexed using frequency divisionmultiplexing, and the CQI reference signal transmission is transmittedfrom the base station at a CQI reference signal transmission period; anda transmitter to transmit to the base station a report determinedaccording to the two CQI reference signal resource elements; wherein thevalue of the subframe offset is within a range from 0 ms to(T_(CQI-RS)-1) ms, where T_(CQI-RS) denotes a CQI reference signaltransmission period, and the report comprises a rank indicator, achannel quality indicator, and a precoding matrix indicator, wherein:the subframe comprises the resource block and a last orthogonalfrequency division multiplexing symbol in the resource block conveys atleast one of the two CQI reference signals; the CQI reference signaltransmission period is cell-specific and a CQI reporting period or a PMIreporting period is user equipment-specific; the two channel qualityindicator reference signals are repeated at a period of 5 subframes, 10subframes, 20 subframes, 40 subframes, or 80 subframes; and the CQIreference signal transmission period is 1 ms, 5 ms, 10 ms, 20 ms, 40 ms,or 80 ms.